1. Technical Field
This disclosure is directed to electronic circuits, and more particularly, to digital-to-analog converter (DAC) circuits.
2. Description of the Related Art
A digital-to-analog converter (DAC) is an electronic circuit configured to convert a digital input word (or code) into an analog output voltage. A differential DAC is configured to convert a digital input word into a differential analog output voltage. For an N bit differential DAC, the output voltages may be expressed as follows:
            V      p        =          VREFB      +                        (                      VREFT            -            VREFB                    )                ⁢                              ∑                          i              =              0                                      N              -              1                                ⁢                                    b              ⁡                              (                i                )                                      ×                          2              i                                                      V      n        =          VREFB      +                        (                      VREFT            -            VREFB                    )                ⁢                              ∑                          i              =              0                                      N              -              1                                ⁢                                                    b                ⁡                                  (                  i                  )                                            _                        ×                          2              i                                          where VREFB and VREFT are reference voltages, and where each b(i) for i=0 to N−1 are the digital input bits.
Various DAC implementations, both differential and single ended, are possible. One type of DAC implementation is a resistor string DAC, or R-DAC, which utilizes one or more networks of series-coupled resistors. A simple R-DAC configured to convert an N-bit code into an analog voltage can be implemented using a series string of 2N resistors and an array of switches. Using the digital code, a single one of the switches may be closed to tap a point on the string of 2N resistors. Thus, for an exemplary embodiment in which N=12, a string of 4096 (212=4096) resistors may be coupled in series, and the output voltage may be generated by tapping one point in the resistors string by closing a corresponding switch according to a digital code.
A segmented R-DAC may be similar to the simple R-DAC described above, but may divide the resistor string into segments. For example, an R-DAC may be implemented with two segments, of M bits and L bits, where N=M+L. A first segment may have 2M resistors, while a second segment may have 2L resistors. Thus, using the example above where N=12, a segmented R-DAC can be implemented with M=6 and L=6. Thus, each segment would include 26=128 resistors. Thus, a segmented 12-bit R-DAC can be implemented using 256 resistors. A segmented R-DAC may include buffers between the segments to combat the effects of loading.